Surgical fixation plates are used in many procedures to mend, align, and alter compression of patients' bones. These plates are primarily secured to the patient's bones by a plurality of fasteners such as screws. Proper orientation and alignment of fasteners and secure surgical fixation of the plates is crucial to avoiding future complications after implantation. This is especially the case for cervical spine locking-plates, such as sold by SYNTHES Spine. These plates are used for long term, intravertebral fixation, bone-fragment fixation, and anterior decompression in the cervical region of the spine. Locking plates enable secure monocortical implantation, meaning that their screws need only penetrate the anterior bone cortex. In conventional plates, screws must pass through both the anterior and posterior bone cortices to attain sufficient support. In passing through both cortices, conventional plates risk penetrating the spinal chord.
Surgeons implanting vertebral plates operate within a fine margin of error. Fairly little vertebral bone is available for setting fasteners. Each plate hole should coaxially align with its screw, i.e., each plate hole has an axis that must align with the screw axis. Otherwise, screws do not seat correctly with the plate. Thus, misalignments can potentially damage tissues, including the spinal cord, or lead to improperly secured plates.
Locking plates in particular demand precise fastener alignment. Cervical locking plates are generally about 2 mm thick. Some screw holes in these plates are inclined by 12° to the surface of the plate to permit optimal screw placement in the cervical region of the spine.
Anchor screws secure the locking plate to the vertebral body. Anchor screws have hollow, longitudinally slotted expansive heads that must fit snugly within a plate's screw hole. These screws are externally threaded to secure to the vertebral bone and the plate. These screws are also threaded internally from their head through a shallow portion of their shaft. Once a surgeon implants an anchor screw, he or she screws a small locking screw into the head of the anchor screw. This locking screw expands the head of the anchor screw so that the head presses outwardly against the locking plate's hole for a compression fit. This compression fit locks the screw in place and creates a solid coupling between the plate and the screw, preventing motion between them and preventing the screw from backing out from the plate, which may damage the esophagus.
This locking mechanism demands extremely precise screw alignment. If the holes drilled in the bone prior to anchor screw insertion are misaligned or off center, anchor screws and locking-plate holes will not seat correctly. Forcing a misaligned anchor screw into the plate hole can collapse the expansive head and prevent insertion of a locking screw. Thus, accurate drill guides for use in drilling the screw hole into the bone are critical to successful operations.
Known drill guides for locking plates, such as disclosed in a SYNTHES Spine catalog dated 1991, are generally a cylindrical tube shaped to receive and guide a drill bit. Most known guides also have a handle. A tip of the tube is shaped to slide into screw holes. A shoulder near the guide tip rests against a modest countersink in the screw hole to limit the guide's insertion into the hole. Constant axial pressure against the plate is required to maintain the guide in the hole, although it is sometimes beneficial to limit unnecessary pressure against the spine during drilling. Also, a clearance between the tip of the guide and the hole is provided to ease insertion into the hole. Due to this clearance, the diminutive thickness of the plate, and the small size of the countersink, an amount of angular play exists in this system. Other similar guides, though shown with femur fixation-plates, are disclosed in U.S. Pat. Nos. 2,494,229, and 5,417,367.
A more accurate drill guide is sold by SYNTHES Spine and shown in its catalog dated 1995, in which angular play is reduced and which does not require a constant force against the plate. This drill guide has an expanding collet formed with a plurality of fingers disposed coaxially about a drill guide sleeve. The sleeve is conical, and when it is slid forward, it spreads the collet fingers to lock them against the inside walls of a screw hole in a cervical spine locking plate. A scissoring handle linked to the collet and the sleeve controls the relative forward and backward motion therebetween.
At the forward tip of the drill guide, the collet has a neck, designed to press against the inside walls of the screw hole. Adjacent this neck is a radially extending rim, which, in a naturally assumed contracted position, has a diameter slightly larger than the screw hole, providing an interference fit. As a surgeon inserts the tip of the collet into the screw hole, the greater diameter of the rim provides a surgeon with a detectable snap and decreased resistance to insertion of the collet as the rim passes to the far side of the hole. To extract the collet from the screw hole, the surgeon must apply a slight force to pry the rim back through the smaller diameter walls of the hole, as these force the rim to contract to the smaller diameter.
A problem frequently arises when using this drill guide during surgery. Once the plate has been carefully positioned in the desired implantation position within the incision, when the surgeon attempts to remove the drill guide from the bone plate, the collet rim often catches on the plate. This catching prevents the drill from releasing the plate, and the surgeon often pulls the plate out of the incision along with the drill guide. As a result, any temporary fixation pins that were holding the plate to the bone could be stripped out of the vertebra, weakening the supporting bone structure, or in the best scenario, the plate would merely become misaligned with previously drilled holes. Even if the plate only becomes misaligned, however, careful realignment of the plate is required before the implantation procedure can continue.
Due to the precise nature of the relationship between the dimensions of screw hole and the rim and neck of the collet, the above problem cannot be avoided by simply using a particular drill guide in combination with any available plate that has larger screw holes. The drill guide and its corresponding locking plates are precisely size-matched and are sold in kits. A drill guide of this type cannot adequately lock and function as a guide with available plates with differently sized holes than those for which the guide was designed. Slightly large holes, for instance, permit excessive play between the plate and the guide, even when the guide is expanded.
Thus, a drill guide is needed that can disengageably lock to a surgical plate fastener hole, but without catching as the drill guide is extracted therefrom.